VAN 121: Veterinary Gross Anatomy-II (2+2)

OSSEOUS LABYRINTH

• The osseous labyrinth is excavated in the petrous part of the petrous temporal bone, medial to the tympanic cavity. It consists of three parts -a middle vestibule, an anterior cochlea and posterior semicircular canals.
• The vestibule with the cochlea in front and semicircular canals behind. It is 5 to 6 mm. long. Its lateral wall separates it from the tympanic cavity and in it are the fenestra vestibuli and fenestra cochleae. Its medial wall corresponds to the fundus of internal acoustic meatus. Internally it is crossed by an oblique ridge the crista vestibuli, which separates the two recesses,
• an anterior smaller recess - recessus sphericus, lodging the saccule and a posterior larger depression - recessus ellipticus lodging the utricle. The two diverging branches of crista vestibuli below include between them the small recess- recessus cochlearis. The recessus have number of minute foramina for passage of nerve fibres to the saccule, utricle, semicircular ducts and cochlear duct. The posterior part presents the four openings of the three semicircular canals. The anterior part of the vestibule shows an elliptical opening leading into the scala vestibuli of the cochlea. The orifice of the aqueduct of the vestibule lies below the recessus ellipticus. The aqueduct extends to the posterior portion of the petrous temporal bone; it transmits a vein and contains a tubular prolongation of the membranous labyrinth that is termed the ductus endolymphaticus.
• The semicircular canals are three in number and are behind and above the vestibule. Each canal is about two thirds of a circle, one end of which is slightly enlarged to form the ampulla. They are at right angles to each other and are designated dorsal, posterior and lateral. They communicate with the vestibule by four openings, as the inner end of the dorsal and the upper end of the posterior unite together and the ampullated ends of dorsal and lateral have a common orifice. The dorsal and posterior canals are nearly vertical and the lateral canal is horizontal.

• The bony cochlea resembles snail-shell and forms anterior part of the bony labyrinth. It is in the form of a blunt cone the base of which corresponds to the anterior part of internal acoustic meatus and the apex or cupola is directed outward, forward and downward. It consists of a spiral canal that forms two and half runs around a bony process, the modiolus. The modiolus is a conical shaped central axis. Projecting from the latter like the thread of a screw is a thin plate of bone, the lamina spiralis that extends about halfway into the canal thus dividing it incompletely into two passages an upper scala vestibuli and a lower scala tympani. In life, a membrane, the membrana basilaris extends from the free margin of the lamina to the lateral wall of the cochlea and thus completes the two scala but they communicate through an opening at the apex called helicotremma. The modiolus is traversed by an axial canal extending from the base to the apex and a spiral canal is present along the attached border of lamina spiralis.
• The base of the cochlea shows three openings:
  • The fenestra cochleae which communicates with the tympanic cavity and in recent state is closed by the secondary tympanic membrane.
  • An elliptical opening to the vestibule.
  • The aperture of the aqueduct of cochlea leading to a minute funnel shaped canal, which opens into the ventral surface of the petrous part of the temporal bone. It transmits a small vein to join the ventral petrosal sinus and also establishes communication between the subarachnoid space and the scala tympani.